Such systems typically consist of a module which performs the actual measurement of the fuel flow and, in case of measuring systems with fuel recirculation, in an additional conditioning module via which it is set that the fuel recirculated from the engine is recirculated into the feed line at a position downstream of the device for flow measurement. The fuel consumption-measuring device in particular consists of a flow meter as is described in DE-AS 1 798 060. The flow meter is an electronically controlled flow measuring device with an inlet and an outlet, between which a rotary displacer in the form of a gear pump, as well as a piston provided in a line parallel to the displacer, are arranged in a measuring chamber. The displacement of the piston is measured in the measuring chamber by an optical sensor to determine the flow volume. The rotational speed of the gear pump is continuously readjusted on the basis of this signal so that the piston is always returned to its initial position, if possible. The flow in a predefined time interval is calculated from the number of rotations or partial rotations of the gear pump measured by an encoder and the known feed volume of the gear pump during one rotation. Return flows from the engine cannot, however, be measured with such systems.
These systems for fuel consumption measurement with a conditioning means are arranged, for example, upstream of a high-pressure fuel pump of a common rail system with a plurality of injection valves. These are closed circuits. It is alternatively basically conceivable to provide a return line to the tank and to arrange a second flow meter therein so that the fuel consumption can be calculated from the difference between the two flow meters. It has been found, however, that such systems do not provide sufficiently exact results due to the very large volumes returned which may possibly be ten times and in extreme cases up to one hundred times the fuel consumption.
Systems for fuel consumption measurement have therefore become known, such as is disclosed, for example, in DE 197 81 795 T1. The system described therein merely comprises one flow meter that is arranged in the feed line leading from the tank to the consumer. A line branches upstream of the flow meter, which leads back to the tank and in which a pressure regulating relief valve is arranged via which the pressure in the feed line can be adjusted. A first return line branches immediately upstream of the consumer and opens into the feed line downstream of the flow meter. Both return lines are combined in a heat exchanger so that the warmer fuel from the first return line is cooled by the colder fuel from the second return line, so that the temperature of the fuel fed from the first return line approximately corresponds to the temperature of the fuel in the feed line. A pressure reducer is additionally provided in the feed line upstream of the opening of the first return line and downstream of the flow meter. Another feed pump is arranged downstream of the opening of the first return line. The flow meter measures the fuel consumption with good accuracy since the fuel pressure and the temperature in the second recirculation circuit and in the first recirculation circuit can be maintained to be substantially equal.
Similar solutions are described in EP 0122105 A1 and in WO 2005/005935 A1.
Problems occur in operational states, however, in which the returned fuel flow is larger than the fed fuel flow. Such a state may occur, for example, when a Diesel engine is started or during the transition from a full load to idle running. The pressure reducer arranged in the feed line prevents a return flow towards the tank, whereby an undesired increase in pressure is caused in the return line. This influences the performance of the internal combustion engine and may even result in damage to aggregates.
It is also common for modern internal combustion engines to use a pump regulation in which the feed volume is adjusted to the expected fuel consumption in order to save energy. The regulation of the pump, however, causes pressure changes in the feed line, wherein the pressure regulating valves at the same time try to create a constant pressure in the feed line and the return line during the measurement. This influences the internal combustion engine which may result in a superposition with the pressure regulation of the internal combustion engine, which may lead to errors in engine management.